The Churchill Barriers are four concrete causeways built in the Second World War to seal the eastern approaches to Scapa Flow, after a U-boat slipped through and sank HMS Royal Oak in 1939. Much of the labour came from Italian prisoners of war held on the islands. The road south from Kirkwall to South Ronaldsay now runs along them, past the rusting hulks of the blockships that were scuttled to close the channels.
